Genotype-environment interactions for behavioral development are ubiquitous and confound the study of effects of early experience on later behavior. However, there are regular patterns for some of these strains by treatment interactions. One such is the wide responsiveness of the C57BL/10 strain to a variety of preweaning treatments, and for a number of behavioral and biological phenotypes. With fostering to mouse mothers as the early experience and aggression as the behavior, this developmental lability appears to require either exposure to the C57BL/10 uterine environment and/or the presence of the C57BL/10 X-chromosome. Also, fostering to rat mothers or to a rat aunt affects the development of aggressive behavior in the C57BL/10. This and other findings suggests a role of maternal behavior in the effects of early experience on aggression. Thus, both biological and environmental mechanisms for the C57BL/10 lability have been explored. Based on these, experiments are proposed to determine: (1) whether or not a variety of early experiences with effects on C57BL/10 behavior require either the strain's uterine environment and/or X-chromosome; (2) whether or not a number of behaviors responding in C57BL/10 mice to preweaning treatments require either the strain's uterine environment and/or X-chromosome; (3) whether the uterine environment or X-chromosome is the crucial factor required for the C57BL/10 lability; and (4) what is the biological mechanism of the C57BL/10 lability. The object of this proposal is a systematic and detailed investigation of the interaction of early experience with genetic variables in an attempt to develop a more complete understanding of the mechanisms between nature and nurture in the development of behavior.